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Arterial aneurysm. Coloured X-ray angiograms of sections through the abdomen and legs of a 72-year-old patient, showing the presence of calcified atherosclerotic plaques that have led to an aneurysm of the popliteal artery. An aneurysm is a blood-filled dilation in a blood vessel wall. It is caused by a weakening of the vessel wall, which may be caused by hardening of the vessel (atherosclerosis), high blood pressure or injury. If left untreated an aneurysm can rupture, which is often fatal.